The present invention relates to an inverter circuit with gate to source threshold voltage compensation. More specifically, the present invention relates to an inverter circuit that at least partially compensates for variations in threshold voltages caused by manufacturing tolerances.
Field Effect Transistors, also known as Metal Oxide Semiconductor (MOS) transistors, are frequently used in numerous types of electronic circuits. Theses transistors form the basic components of a semiconductor integrated circuit and are typically formed on a doped substrate. On the substrate is deposited a metal oxide layer that is etched to form the gate source and drain electrodes. However, the miniaturization of MOS transistors makes it is difficult to consistently manufacture batches of semiconductor integrated circuits with exactly the same substrate doping levels that have exactly the same etched dimensions and positions of gate source and drain electrodes.
The inconsistencies in manufacturing MOS transistors (manufacturing tolerances), although small, result in variations in the transistor's gate to source threshold voltage Vth. Consequently, at the time of manufacture the exact threshold voltage Vth for a MOS transistor is generally unknown and only it's value+/−a manufacturing tolerance is known. This threshold voltage Vth is the minimum voltage required to make a MOS transistor conduct and can affect the desired output for even relatively simple circuits. For instance, one simple circuit is a pulse generator comprising a two input OR gate with a first input coupled to a directly to a clock and a second input coupled to the clock through three inverters. The inverters cause a delay in the clock reaching the second input of the OR gate and this delay affects the width of the generated pulse at the output of the OR gate. If the pulse generator is formed by MOS transistors then the inverter's response time, due to the threshold voltage Vth variations, can result in the width of the generated pulse varying considerably for different batches of manufactured pulse generators.